1. Field of the Invention
The present invention relates to a handpiece for dental use, and more particularly, to a handpiece to be used by a dentist for cutting or drilling a root canal of a tooth.
2. Description of the Related Art
In curing a root canal of a tooth, a cutting tool such as a dental reamer or a dental file with an edge is generally used to form a root canal. These cutting tools are provided with a spiral cutting edge, and the edge portion is inserted into a root canal, and then, the back and forth movements or rotational movements of the cutting tools by changing the cutting tools corresponding to the shape of the root canal allows the root canal to be tapered. To cut a root canal, a handle of a cutting tool is held for manual operation, or a cutting tool is connected to a dental handpiece for using rotationally driving force of an engine such as a motor.
Since a dental reamer has a large lead, that is, a relatively gentle spiral cutting edge, it is mainly used for cutting by rotational movements. In contrast, a dental file has a spiral cutting edge having a lead smaller than that of the dental reamer; therefore, it is mainly used for cutting through back and forth movements like a file. Further, the dental file is often used by incorporating the back and forth movements as well as some rotational movements at the same time, which allows a root canal to be cut by the back movement and debris generated by the cutting to be carried away to outside of the root canal through spiral channels of the edge portion. xe2x80x9cThe cuttingxe2x80x9d means that an edge portion of a cutting tool enters the wall of a root canal to some extent, and a part of the wall is scraped off. However, if the edge portion of the cutting tool excessively enters the wall, it becomes impossible to perform the cutting operation.
A dental handpiece heretofore in use for operating these cutting tools is sometimes capable of driving the tools by back and forth movements, by rotational movements, or by both the back and forth movements and the rotational movements. A dental handpiece is generally driven by an engine such as a motor, thus, a type of a transmission which transmits the driving force to the cutting tools and a manner that the transmission rotates decide the movement of the cutting tool either the back and forth movements or the rotational movements only, or both the back and forth movements and the rotational movements.
In the cutting with driving force from an engine, the rotation of a motor is transformed to the back and forth movements or the rotational movements and is applied to a cutting tool through a transmission, and the cutting tool cuts the wall of a root canal, and the cutting tool is pushed toward a root apex to form the root canal. Therefore, in the cutting operation, there is a problem that excessive force is applied to the cutting tool, even if the tool size is larger than that of a root canal to be cut, the cutting tool proceeds deep into the root canal while entering the wall thereof, and then the cutting tool is deformed or broken due to the excessive cutting.
Further, as shown in FIGS. 9A and 9B, there is a problem of creating a ledge in the root canal. FIGS. 9A and 9B show that a ledge is formed in a root canal 1a of a tooth 1 to be cured. As illustrated in FIG. 9A, the root canal 1a is curved. When a cutting tool 2 inserted into the root canal 1a can not follow the curved root canal 1a smoothly due to the insufficient resiliency, and the further rotational movements or back and forth movements are added to the cutting tool 2, a ledge shown in FIG. 9B is formed. As a result, it becomes impossible to form a root canal along the curve.
In addition to the above, as shown in FIGS. 10A and 10B, the root canal 1a is tapered toward its root apex, as illustrated in FIG. 10A, if the diameter of the cutting tool 2 is too large in relation to that of the root canal, the cutting tool 2 must be changed to a thinner cutting tool 2xe2x80x2 to cut the root canal. Generally, it is required to change the cutting tools several times to form one root canal. However, in a dental handpiece driven by a motor in the conventional technology, it is difficult to appropriately know the timings to change the cutting tools. Therefore, when the timing to change the cutting tools is advanced to prevent the breakage of the cutting tool or the generation of the ledge, there is a problem that the formation of the root canal becomes insufficient since the cutting tool in operation is too thin.
When a ledge is formed, a cutting tool will not cut along a root canal, which causes a problem where a large force is applied to the cutting tool. When a thick cutting tool is used to cut a thin root canal also, a large force is applied to the cutting tool. That is, the above-mentioned problems arise because a large load that is applied to a cutting tool cannot be controlled; therefore, in case that an excessive load is applied to a cutting tool, it is necessary to suspend the cutting operation.
In consideration of the above problems, a dental handpiece with a mechanism for controlling torque in a driving connection has been proposed in Japanese Patent Application Laid-open Heisei 10-314185. In this dental handpiece, a coupling for preventing an excessive load is mounted in the driving connection to drive a cutting tool, and when the excessive load is applied to the cutting tool, resultant slippage produced in the coupling suspends the cutting operation by the cutting tool.
However, since the coupling for preventing the excessive load is mounted at a place in the driving connection, which is far from the cutting tool, it is difficult to detect the change of the load applied to the cutting tool itself. In other words, the load applied to the coupling is the sum of a load applied to the cutting tool and a load applied to a transmission including a drive shaft and a bearing, thus it is difficult to distinguish whether an edge portion of the cutting tool excessively enters a wall of a root canal or the resistance of the drive shaft increases. Therefore, even if a threshold level of the load that the cutting tool should be slipped is set, it is practically impossible to appropriately judge the timing to change the cutting tools or the like.
It is therefore an object of the present invention to improve the above dental handpiece and provide a dental handpiece to securely detect the load applied to the handpiece itself, and when an excessive load is applied to the handpiece, the cutting operation is suspended. To accomplish the above objective, a dental handpiece according to the present invention comprises: a head; a support mounted in the head to support a cutting tool; and a transmission mounted in the head to transmit driving force to the support; wherein the support and the transmission are slidably connected with each other.
In the above dental handpiece, preferably, the support is cylindrical and is provided with a plurality of inclined projections that are arranged in a longitudinal direction of an outer periphery of the cylinder; the transmission is provided with engaging projections with inclined faces parallel to inclined faces of the plurality of inclined projections; and the transmission is provided with resiliency that allows the engaging projections movable in a direction departing from the inclined projections.
In the above dental handpiece, it is preferable to provide a plurality of inclined projections with inclined faces different from those of the inclined projections at positions different from those of the inclined projections; and the transmission engages with one selected from two groups of inclined projections. In this construction, it is preferable that one of the groups of inclined projections engages with the transmission only in one direction and is always slidable in relation to the transmission in a direction opposite to the one direction. In addition, another group of projections engage with the transmission only in a direction reverse to the above. As a result of the above construction, the transmission is preferably movable to a desired position by changing an angle relative to the support.
Besides, it is possible to form the dental handpiece according to the present invention in such a manner that the support is cylindrical and is provided with a ratchet gear at an outer periphery thereof; the dental handpiece further comprises an inner ratchet engaging with the ratchet gear, a rotation transmitting means for rotating the inner ratchet and a rotationally driving means for driving the rotation transmitting means; and the inner ratchet is resiliently displaceable in a direction that an engagement with the ratchet gear is released. The rotation transmitting means and the rotationally driving means may be bevel gears that engage with each other.
With the dental handpiece described above, the wall of a root canal is cut by the cutting tool with rotational movements and back and forth movements. When the cutting tool cuts only one portion (a portion out of convex canal) and a ledge is about to be formed, the load applied to the cutting tool becomes large, so that the cutting tool slips, which prevents further cutting operation. In addition, when the diameter of a root canal becomes small, and the diameter of the cutting tool becomes large relative to the root canal, load applied to the cutting tool becomes large, so that the cutting tool slips, which prevents further cutting operation. As a result, it is possible to know the timing that the cutting tool should be changed. Like this, the dental handpiece of the present invention is characterized in that the rate that the energy from a motor is transmitted can be changed in accordance with the load applied to the cutting tool. In other words, when the load becomes large, slippage occurs between the support and the transmission, which intentionally weaken the motion of the cutting tool. Thus, in this invention, rather than by a driving source such as a motor, the slippage is produced through a mechanical structure such as the connection between the support and the transmission.
Without the slippage in dental handpieces in the past, the above problems frequently occur, and the control mainly relies upon the sense of a dentist, which is a burden to the dentist. With the present invention, mechanical control is achieved, resulting in a safe enlarging operation of a root canal. Especially, with the construction according to the present invention, a slipping mechanism is mounted to a head near the object to be cured, thus it becomes possible to quickly detect an excessive increase of load due to catching of a cutting tool.